For many manufactured products and components it is important that surface characteristics such as form and shape are within defined tolerances. Metrological instruments are known which measure such surface characteristics for quality control purposes. These metrological instruments must be manufactured to a high precision in order to minimise the effect of systematic errors introduced by the metrological instrument on measurement readings.
EP-A-0240151 discusses a metrological instrument which measures the form of a workpiece mounted on a turntable. In particular, a stylus gauge is supported relative to the turntable and generates a signal representative of the deflection of the elongate stylus along a measurement direction as the stylus follows a surface of the workpiece on rotation of the turntable about a spindle axis. The stylus gauge is mounted on an attitude arm which is rotatable about a pivot axis so that the stylus can be switched between a substantially vertical configuration and a substantially horizontal configuration while generally maintaining the stylus tip in the same position. The stylus gauge also includes a stylus orientation mechanism for rotating the measurement direction.
A problem with Instruments such as that discussed in EP-A-0240151 is that imperfections in the manufacture of the stylus gauge and the stylus can lead to a loss of the correct alignment of the measurement direction at the stylus tip and the spindle axis. Previously, manual adjustments have been provided for correcting any such misalignment. However, these manual adjustments must be performed each time the attitude of the stylus or orientation of the measurement direction is changed to achieve optimal accuracy.